This invention is directed to an apparatus for shaping an electron or ion beam, particularly for shaping the cross section of a focused electron or ion beam for lithographic exposure and the like. The apparatus provides a selection of sizes and shapes of the focused beam spot on the target.
Electron and ion beams are used to expose photolithographic sensitive resist material on a semiconductor wafer so that after removal of the nonmasking portions of the resist, the wafer can be doped, usually by ion implantation. As beam spot resolution improves, finer lines can be drawn, with closer spacing but such small spots require considerable time to expose large areas. The writing speed of conventional electron beam lithography systems, in which the demagnified image of a thermionic electron source serves as the writing spot, is often inadequate for submicron circuit fabrication purposes.
An alternative approach to submicron electron and ion beam lithography is to use shaped spots. Several investigators have shown that variable shaped spots with good edge resolution can increase the writing speed potential dramatically as outlined in the following papers: M. G. R. Thomson, R. J. Collier, and D. R. Herriott, Journal of Vacuum Science Technology 15, 891 (1978); and H. C. Pfeiffer, Journal of Vacuum Science Technology 15, 887 (1978). In fact, for minimum feature sizes of 1-2 microns, this approach has been proven in an actual production environment (see: G. J. Guiffre, J. F. Marquis, H. C. Pfeiffer, W. Stickel, 15th Symposium on Electron, Ion and Photon Beam Technology, Boston (1979).
A drawback of some spot-shaping methods is the complexity of their electron-optics. As described by M. G. R. Thomson, et al and H. C. Pfeiffer, supra, spot-shaping columns with aperture-to-aperture imaging consist of 5 or 6 lenses and 2 to 4 sets of beam alignment coils. Two of the lenses provide variable aperture imaging, while the remaining lenses serve to demagnify and project the shaped spot onto the target plane. Alignment coils are required since misalignment results in loss of the beam, as compared to Gaussian systems where beam misalignment only results in spot displacement.
A drawback of other spot-shaping methods is vignetting. In systems with aperture-to-aperture shadow casting, the beam does not return to the center of the final lens and, therefore, the spot current density varies as spot size is changed. This type of vignetting is seen in Hidekazu Gotou, Japanese, Pat. No. 127,682.
Therefore, there is need for a beam system which provides for shaped aperture exposure of resist, with the apparatus being simple in configuration so that it can be readily and economically mechanized and controlled and is without vignetting such that the spot current density remains independent of spot size.